1. Field of the Invention
The present invention relates to a lamination type LC filter.
2. Description of the Related Art
Conventionally, among filters used in portable telephones and other electronic apparatuses, a lamination type LC filter is known. The lamination type LC filter is a chip electronic component including inductor electrodes and capacitor electrodes that constitute a resonance circuit and are laminated through dielectric sheets. Such a component is suitable for miniaturization.
Such a lamination type LC filter is shown in FIGS. 14 to 16. FIG. 14 is an exploded perspective view of a conventional lamination type LC filter, FIG. 15 is a perspective appearance of the LC filter, and FIG. 16 is an equivalent circuit diagram of the LC filter.
As shown in FIG. 14, the lamination type LC filter 1 includes a dielectric sheet 3a on the surface of which an internal ground electrode 2 is provided, a dielectric sheet 3b on the surface of which inductor electrodes 4a and 4b are provided, a dielectric sheet 3c on the surface of which an input capacitor electrode 5a and an output capacitor electrode 5b are provided, a dielectric sheet 3d on the surface of which resonance capacitor electrodes 6a and 6b are provided, a dielectric sheet 3e on the surface of which an internal ground electrode 7 is provided, and a dielectric sheet 3f constituting an external layer.
Each internal electrode is made of Ag, Pd, Cu, Ni, Au, or Agxe2x80x94Pd, and is formed via printing, sputtering, or evaporation. Furthermore, a dielectric powder mixed and kneaded together with a binder, is made into sheets which are used as the dielectric sheets 3a to 3f. 
The internal ground electrode 2 is formed on substantially the entire area, excluding the left and right end portions, of the surface of the dielectric sheet 3a, such that a first end portion 2a of the internal ground electrode 2 is exposed at the front of the dielectric sheet 3a, and a second end portion 2b is exposed at the rear thereof.
The inductor electrodes 4a and 4b include a spiral coil of about two turns, the inductor electrode 4a is provided at a left portion of the dielectric sheet 3b, and the inductor electrode 4b is provided at a right portion of the dielectric sheet 3b. A first end portion 4a1 of the inductor electrode 4a is connected to the resonance capacitor 6a through a via hole, and a second end portion 4a2 of the inductor electrode 4a is connected to the internal ground electrode 2 through another via hole. In the same way, the first end portion 4b1 of the inductor electrode 4b is connected to the resonance capacitor 6b through a via hole, and the second end portion 4b2 of the inductor electrode 4b is connected to the internal ground electrode 2 through another via hole.
In the input capacitor electrode 5a, a first end portion 5a1 is exposed at a left portion of the dielectric sheet 3c, and a second end portion 5a2 has a large area and faces the resonance capacitor electrode 6a through the dielectric sheet 3d. In the same way, in the output capacitor electrode 5b, a first end portion 5b1 is exposed at the right portion of the dielectric sheet 3c, and a second end portion 5b2 has a large area and faces the resonance capacitor electrode 6b through the dielectric sheet 3d. 
Both of the resonance capacitor electrodes 6a and 6b are rectangular and have longer sides extending in the direction from the front to the rear of the dielectric sheet 3d, the resonance capacitor electrode 6a is provided at a left portion of the dielectric sheet 3d, and the resonance capacitor electrode 6b is provided at a right portion of the dielectric sheet 3d. As described above, the resonance capacitor electrode 6a is connected to the first end portion 4a1 of the inductor electrode 4a through a via hole and faces the input capacitor electrode 5a. In the same way, the resonance capacitor electrode 6b is connected to the first end portion 4b1 of the inductor electrode 4b through a via hole and faces the input capacitor electrode 5b. 
The internal ground electrode 7 is formed along substantially the entire area, excluding the left and right end portions, of the surface of the dielectric sheet 3e, such that a first end portion 7a is exposed at the front of the dielectric sheet 3e, and a second end portion 7b is exposed at the rear of the dielectric sheet 3e. 
Each dielectric sheet having the above-described construction is put one on top of another in order, and then they are integrally fired to form a laminated body.
Next, as shown in FIG. 15, external ground electrodes 8a and 8b are formed on the front and rear surfaces of the obtained laminated body, respectively. Furthermore, an external input electrode 9a and an external output electrode 9b are formed on the left and right surfaces of the laminated body, respectively. Each external electrode is formed by means of coating, baking, sputtering, and evaporation. Then, a first end portion 2a of the internal ground electrode 2 and a first end portion 7a of the internal ground electrode 7 are connected to the external ground electrode 8a, and the second end portion 2b and the second end portion 7b of the internal ground electrode 7 are connected to the external ground electrode 8b. Furthermore, the first end portion 5a1 of the input capacitor electrode 5a is connected to the external input electrode 9a, and the first end portion 5b1 of the output capacitor electrode 5b is connected to the external output electrode 9b. 
The lamination type LC filter 1 described above constitutes the equivalent circuit diagram of a bandpass filter as shown in FIG. 16. That is, an inductance L1 generated by the inductor electrode 4a and a capacitance C1 generated between the resonance capacitor electrode 6a and the internal ground electrode 7 are connected in parallel and constitute a parallel LC resonance circuit Q1 on the input side. In the same way, an inductance L2 generated by the inductor electrode 4b and a capacitance C2 generated between the resonance capacitor electrode 6b and the internal ground electrode 7 are connected in parallel and constitute a parallel LC resonance circuit Q2 on the output side. Then, a mutual inductance M is formed between the inductance L1 and the inductance L2, and the parallel LC resonance circuits Q1 and Q2 are magnetically coupled.
Furthermore, a capacitance formed by a capacitive coupling between the second end portion 5a2 of the input capacitor electrode 5a and the resonance capacitor electrode 6a is connected in series to the parallel resonance circuit Q1 to constitute an input adjustment capacitance C3. In the same way, a capacitance formed by a capacitive coupling between the second end portion 5b2 of the output capacitor electrode 5b and the resonance capacitor electrode 6b is connected in series to the parallel resonance circuit Q2 to constitute an output adjustment capacitance C4.
Here, in a perspective view, when the above-described lamination type LC filter 1 is seen in the direction of an arrow A in FIG. 15 (from the top), the input capacitor electrode 5a located above the inductor electrode 4a is disposed inside the inductor electrode 4a, the resonance capacitor electrode 6a is disposed so as to completely cover the inductor electrode 4a, and the internal ground electrode 7 is disposed so as to completely cover the inductor electrode 4a. On the other hand, the internal ground electrode 2 below the inductor electrode 4a is disposed so as to completely enclose the inductor electrode 4a. 
In the same way, the second end portion 5b2 of the output capacitor electrode 5b located above the inductor electrode 4b is disposed inside the inductor electrode 4b, the resonance capacitor electrode 6b is disposed so as to completely cover the inductor electrode 4b, and the internal ground electrode 7 is disposed so as to completely cover the inductor electrode 4b. On the other hand, the internal ground electrode 2 located below the inductor electrode 4b is disposed so as to completely enclose the inductor electrode 4b. 
In keeping with the trend toward producing smaller and more low-profile electronic components, in the construction of conventional lamination type LC filters, the input-output capacitor electrodes, the resonance capacitor electrodes, the internal ground electrodes, and other elements, which constitute capacitance are located close to the inductor electrodes. Also, the magnetic field generated by the inductor electrodes is interrupted by these internal electrodes, and the Q characteristics of the inductor electrodes deteriorate. Consequently, these disadvantages prevent making the lamination type LC filters smaller and more low-profile.
In order to overcome the problems described above, preferred embodiments of the present invention provide a lamination type LC filter in which the interruption of a magnetic field generated by the inductor electrodes is prevented by the other internal electrodes constituting capacitance, thus producing greatly improved filtering characteristics.
A lamination type LC filter according to a preferred embodiment of the present invention includes a laminated body in which inductor electrodes, capacitor electrodes, and internal ground electrodes constituting a resonance circuit are laminated through dielectric sheets. In the lamination type LC filter, the inductor electrodes preferably have a bent shape or a curved shape. A first end portion of the inductor electrodes is located in the vicinity of the central portion of the dielectric sheets, a middle portion thereof is located in the vicinity of the end portion of the dielectric sheets, and a second end portion thereof is located in the vicinity of the central portion of the dielectric sheets. The capacitor electrodes and the internal ground electrodes are preferably strip-shaped in the vicinity of the central portion of the dielectric sheets and are stacked on top of another so as to be substantially parallel to each other.
With the unique construction described above, in a perspective view when the lamination type LC filter is seen from the top, in the inductor electrodes, since the first end portion and the second end portion of the inductor electrodes are located in the vicinity of the central portion of the dielectric sheet and the middle portions of the inductor electrodes are located in the vicinity of the end portion, the portion in which the inductor electrodes overlap with the capacitor electrodes and the internal ground electrodes having the strip-shaped configuration in the vicinity of the central portion is very small. Therefore, a magnetic field generated by the inductor electrodes is minimized by the other internal electrodes.
Furthermore, a lamination type LC filter according to another preferred embodiment of the present invention includes a laminated body in which inductor electrodes, capacitor electrodes, and internal ground electrodes constituting a resonance circuit are laminated through dielectric sheets. In the lamination type LC filter, the inductor electrodes preferably have a bent shape or a curved shape. A first end portion of the inductor electrodes is located in the vicinity of the central portion of the dielectric sheets, a middle portion thereof is located in the vicinity of the end portion of the dielectric sheets, and a second end portion thereof is located in the vicinity of the central portion of the dielectric sheets. The capacitor electrodes and the internal ground electrodes preferably have a strip-shaped configuration in the vicinity of the end portion of the dielectric sheets and are stacked on top of another so as to be substantially parallel to each other.
With the unique construction described above, in a perspective view when the lamination type LC filter is seen from the top, in the inductor electrodes, since the first end portion and the second end portion are located in the vicinity of the central portion of the dielectric sheets and the middle portion is provided in the vicinity of the end portion, the portion in which the inductor electrodes overlap with the capacitor electrodes and the internal ground electrodes having the strip-shaped configuration in the vicinity of the end portion is very small. Therefore, a magnetic field generated by the inductor electrodes is minimized by the other internal electrodes.
Moreover, in a lamination type LC filter according to another preferred embodiment of the present invention, external ground electrodes to be connected to the internal ground electrodes are provided on the side surfaces of the laminated body and the external ground electrodes preferably have substantially the same width as that of the internal ground electrodes.
With above-described unique construction, when the internal ground electrodes are located in the vicinity of the central portion of the dielectric sheets and have a strip-shaped configuration, the external ground electrodes are also located in the vicinity of the central portion so as to have substantially the same width. Therefore, the first end portion and the second end portion of the inductor electrodes are provided in the vicinity of the central portion, and the portion in which the internal ground electrodes overlap with the inductor electrodes, the middle portion of which is provided so as to go around in the vicinity of the end portion, is very small. Therefore, a magnetic field generated by the inductor electrodes is minimized by the other internal electrodes.
Furthermore, when the internal ground electrodes are located in the vicinity of the end portions of the dielectric sheets, the external ground electrodes are also located in the vicinity of the end portions so as to have substantially the same width. Therefore, the first end portion and the second end portion of the inductor electrodes are provided in the vicinity of the central portion, and the portion in which the external ground electrodes overlap with the internal electrodes, the middle portion of which is provided so as to go around in the vicinity of the end portion, is very small. Therefore, a magnetic field generated by the inductor electrodes is minimized by the other internal electrodes.